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Beijing 100085, P.R.China

Rain Measurement with SIR-C/X-SAR

AbstractRain measurements by ground-based radar are unavailable in many parts of the world, especially the oceans. Because many SARS will be flying in space, we (a group headed by Dr. F. Li of JPL) performed an experiment using the SIR-C/X-SAR radars to test the ability of spaceborne SARS to measure rain rates. SAR resolution in rain is degraded by Doppler shifis due to turbulent motion of the rain (Atlas and Moore, 1987). The wide vertical beam required to achieve SAR ground coverage means that one must accept a path-integrated estimate of the rain rate. We used two approaches to rain measurement using the SIR-C/X-SAR radars pointed well off vertical. Over the western Pacific Ocean, we used the strength of the rain echo to estimate rain rate using an appropriate Z-R relation, as do ground-based radars, To separate rain echo from surface echo, one must estimate the surface contribution to the signal. Over the Amazon rain forest where the shadows are obvious we used the reduction of signal in the shadow due to attenuation through a rain storm. THE EXPERIMENT SIR-C was a multi-polarized SAR with frequencies in L band and C band while the X-band X-SAR used only vertical (VV) polarization. The SRL-1 mission was in April 1994, and the SRL-2 mission in September-October 1994. We obtained storm images from both in both the southwest Pacific and in Brazil. We could only turn the radars on a few times, with requests needed about 20 hours in advance for the southwest Pacific passes, The Brazil passes were scheduled for other purposes. For rain forecasts, the JPL team used cloud IR images from the GMS satellite and the Kansas team obtained forecasts of

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Description

Abstract : AbstractRain measurements by ground-based radar are unavailable in many parts of the world, especially the oceans. Because many SARS will be flying in space, we (a group headed by Dr. F. Li of JPL) performed an experiment using the SIR-C/X-SAR radars to test the ability of spaceborne SARS to measure rain rates. SAR resolution in rain is degraded by Doppler shifis due to turbulent motion of the rain (Atlas and Moore, 1987). The wide vertical beam required to achieve SAR ground coverage means that one must accept a path-integrated estimate of the rain rate. We used two approaches to rain measurement using the SIR-C/X-SAR radars pointed well off vertical. Over the western Pacific Ocean, we used the strength of the rain echo to estimate rain rate using an appropriate Z-R relation, as do ground-based radars, To separate rain echo from surface echo, one must estimate the surface contribution to the signal. Over the Amazon rain forest where the shadows are obvious we used the reduction of signal in the shadow due to attenuation through a rain storm. THE EXPERIMENT SIR-C was a multi-polarized SAR with frequencies in L band and C band while the X-band X-SAR used only vertical (VV) polarization. The SRL-1 mission was in April 1994, and the SRL-2 mission in September-October 1994. We obtained storm images from both in both the southwest Pacific and in Brazil. We could only turn the radars on a few times, with requests needed about 20 hours in advance for the southwest Pacific passes, The Brazil passes were scheduled for other purposes. For rain forecasts, the JPL team used cloud IR images from the GMS satellite and the Kansas team obtained forecasts of

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